Method of electrically connecting a microelectronic component

ABSTRACT

A microelectronic unit can include a support structure including a dielectric having oppositely-directed first and second surfaces. A plurality of substantially rigid posts can protrude parallel to one another in a direction beyond the first surface of the support structure. Each post may have a top surface remote from the support structure, and the top surfaces can be substantially coplanar with one another. A microelectronic device having a surface with bond pads can overlie the second surface of the support structure with the bond pad-bearing surface of the microelectronic device facing toward the support structure. Connections can electrically connect the posts with the bond pads.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 11/257,296, filed Oct. 24, 2005; which application is a continuationof U.S. patent application Ser. No. 10/980,381, filed Nov. 3, 2004 andissued as U.S. Pat. No. 7,138,299 on Nov. 21, 2006; which application isa divisional of U.S. patent application Ser. No. 09/707,452 filed Nov.7, 2000, and issued as U.S. Pat. No. 6,826,827 on Dec. 7, 2004; whichapplication is a divisional of U.S. patent application Ser. No.08/885,238, filed on Jun. 30, 1997, and issued as U.S. Pat. No.6,177,636 on Jan. 23, 2001; and which is a continuation of U.S. patentapplication Ser. No. 08/366,236, filed on Dec. 29, 1994, the disclosuresof which are hereby incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates, generally, to interconnectingmicroelectronic devices and supporting substrates, and more particularlyrelates to an apparatus and a method of interconnecting microelectronicdevices to supporting substrates using subtractively created members.

BACKGROUND OF THE INVENTION

Complex microelectronic devices such as modem semiconductor chipsrequire many hundreds of input and output connections to otherelectronic components. These device connections are generally eitherdisposed in regular grid-like patterns, substantially covering thebottom surface of the device (commonly referred to as an “area array”)or in elongated rows extending parallel to and adjacent each edge of thedevice's front surface. The various prior art processes for making theinterconnections between the microelectronic device and the supportingsubstrate use prefabricated arrays or rows of leads/discrete wires,solder bumps or combinations of both, such as with wire bonding, tapeautomated bonding (“TAB”) and flip/chip bonding.

In a wirebonding process, the microelectronic device may be physicallymounted on a supporting substrate. A fine wire is fed through a bondingtool and the tool is brought into engagement with a contact pad on thedevice so as to bond the wire to the contact pad. The tool is then movedto a connection point of the circuit on the substrate, so that a smallpiece of wire is dispensed and formed into a lead, and connected to thesubstrate. This process is repeated for every contact on the chip. Thewire bonding process is also commonly used to connect the die bond padsto lead frame fingers which are then connected to the supportingsubstrate.

In a tape automated bonding (“TAB”) process, a dielectric supportingtape, such as a thin foil of polyimide is provided with a hole slightlylarger than the microelectronic device. An array of metallic leads isprovided on one surface of the dielectric film. These leads extendinwardly from around the hole towards the edges of the hole. Each leadhas an innermost end projecting inwardly, beyond the edge of the hole.The innermost ends of the leads are arranged side by side at a spacingcorresponding to the spacing of the contacts on the device. Thedielectric film is juxtaposed with the device so that the hole isaligned with the device and so that the innermost ends of the leads willextend over the front or contact bearing surface on the device. Theinnermost ends of the leads are then bonded to the contacts of thedevice, typically using ultrasonic or thermocompression bonding, and theouter ends of the leads are connected to external circuitry.

In both wire bonding and conventional tape automated bonding, the padson the substrate are arranged outside of the area covered by the chip,so that the wires or leads fan out from the chip to the surroundingpads. The area covered by the entire assembly is considerably largerthan the area covered by the chip. This makes the entire assemblysubstantially larger than it otherwise would be. Because the speed withwhich a microelectronic assembly can operate is inversely related to itssize, this presents a serious drawback. Moreover, the wire bonding andtape automated bonding approaches are generally most workable with chipshaving contacts disposed in rows extending along the edges of the chip.They generally do not allow use with chips having contacts disposed inan area array.

In the flip-chip mounting technique, the front or contact bearingsurface of the microelectronic device faces towards the substrate. Eachcontact on the device is joined by a solder bond to the correspondingcontact pad on the supporting substrate, as by positioning solder ballson the substrate or device, juxtaposing the device with the substrate inthe front-face-down orientation and momentarily reflowing the solder.The flip-chip technique yields a compact assembly, which occupies anarea of the substrate no larger than the area of the chip itself.However, flip-chip assemblies suffer from significant problems whenencountering thermal stress. The solder bonds between the devicecontacts and the supporting substrate are substantially rigid. Changesin the relative sizes of the device and the supporting substrate due tothermal expansion and contraction in service create substantial stressesin these rigid bonds, which in turn can lead to fatigue failure of thebonds. Moreover, it is difficult to test the chip before attaching it tothe substrate, and hence difficult to maintain the required outgoingquality level in the finished assembly, particularly where the assemblyincludes numerous chips.

As the number of interconnections per microelectronic device increases,the issue of interconnection planarity continues to grow as well. If theinterconnections are not planar with respect to each other, it is likelythat many of the interconnections will not electrically contact theirjuxtaposed contact pads on a supporting substrate, such as a standardprinted wiring board. None of the above described techniques provides acost effective interconnection scheme which guarantees the planarity ofthe interconnections so that each is assured of making an electricalcontact with the contact pads on the opposed supporting substrate.

Numerous attempts have been made to solve the foregoing interconnectionproblems. An interconnection solution put forth in U.S. Pat. No.4,642,889, entitled “Compliant Interconnection And Method Therefor,”issued Apr. 29, 1985, to Grabbe creates an interconnection scheme byembedding wires within each solder column/ball to reinforce the solderthereby allowing higher solder pedestals and more elasticity. Furtherinterconnection solutions put forth include providing a combination ofsolder and high lead solder thereby allowing higher solder pedestals andmore elasticity given the high lead content of the solder, as found inU.S. Pat. 5,316,788, entitled “Applying Solder To High DensitySubstrates,” issued May 31, 1994 to Dibble et al. and U.S. Pat. Nos.5,203,075 and 5,133,495, issued respectively on Apr. 20, 1993 and Jul.28, 1992 to Angulas et al.

U.S. Pat. No. 4,955,523, entitled “Interconnection Of ElectronicComponents,” issued on Sep. 11, 1990, to Calomagno et al. puts forth astill further interconnection technique in which wires are wirebonded tothe contact pads on a first surface, cut to a desired length and thenattached to a second opposing surface by placing each of the wires in a“well” of conductive material, such as solder. While the wires then givea certain amount of compliancy to the structure, this techniqueencounters difficulties in controlling unwanted bending and electricalshorting of the wires prior to and during the coupling step in theirrespective solder wells. Similarly, U.S. Pat. No. 5,067,007, entitled“Semiconductor Device Having Leads For Mounting To A Surface Of APrinted Circuit Board,” issued Nov. 19, 1991, to Kanji et al. disclosesthe use of stiff or deformable lead pins to increase the pin pitch anddeal with problems stemming from thermal coefficient of expansionmismatches between the device and a printed circuit board. Besides thepotential for bending and shorting of the pins as described above, thepins are individually attached to both the device and the printedcircuit board by brazing or soldering making this a time consuming andless than optimum solution from a manufacturing point of view.

U.S. Pat. No. 4,067,104, entitled “Method Of Fabricating An Array OfFlexible Metallic Interconnects For Coupling MicroelectronicComponents,” issued on Jan. 10, 1978 to Tracy uses an additive techniquewhere the interconnections are created by providing a layer ofphotoresist, removing portions of the photoresist and depositing metalwithin the removed portions. By successively following this technique, aplurality of metalized columns are created and coupled to opposingcontact pads on a supporting substrate by a suitable method, such asflip chip bonding, cold welding, diffusion bonding or melting. Thephotoresist is then removed. However, interconnection planarity issuescan become a problem when practicing the invention disclosed in Tracy'104. Further, the strength of each of the interconnection columns maybe impeded due to the joining of the different layers of metal and tothermal cycling fatigue.

One commonly assigned invention, U.S. patent application Ser. No.08/190,779, filed Feb. 1, 1994 and issued as U.S. Pat. No. 5,455,390 onOct. 3, 1995, deals effectively, but specifically differently, with manyof the problems encountered by the prior art. In one embodiment, the'779 application interconnects the device contact pads to the supportingsubstrate terminals by using leads which are coupled to the terminals ina conventional manner, such as by soldering, such that they extendsubstantially side by side. The unconnected ends are then coupled to thecontact pads through the use of predetermined pressure and temperatureconditions. A support layer is then disposed between the device and thesupporting substrate and further surrounds and supports the leads. Thisstructure effectively deals with thermal expansion mismatch and leadshorting problems.

Despite these and other efforts in the art, still further improvementsin microelectronic interconnection technology would be desirable.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for providinginterconnections between a microelectronic device and a supportingsubstrate which substantially obviates many of the problems encounteredby the prior art.

One embodiment of the present invention provides a method of fabricatingan interconnection component for a microelectronic device comprisesproviding a support structure, typically comprised of a flexible butsubstantially inextensible substrate, having a first and a secondsurface, where a conductive sheet is coupled to the first surface of thesupport structure. The conductive sheet is then selectively removed,typically using an etching process, thereby producing a highly planar,cost effective plurality of substantially rigid posts each of whicheventually become the interconnections between the microelectronicdevice and a supporting substrate. The etching process generally firstincludes applying a photoresist layer to the conductive sheet andexposing portions of the photoresist layer to form etch resistantportions and remainder portions. The remainder portions may then beremoved and the conductive sheet may be etched around the etch resistantportions.

A compliant layer may then be provided on the second surface of thesupport structure and a microelectronic device having a plurality ofbond pads may be engaged with the exposed surface of the compliantlayer. The compliant layer is used to substantially accommodate thermalcoefficient of expansion mismatches between the device and a supportingsubstrate when the device is in use. Each bond pad is then electricallycoupled to at least one conductive post. The bond pads and posts may becoupled in a number of different ways, including plating a plurality ofetch resistant conductive leads on either the first surface of thesupport structure or the conductive sheet such that the leads aresandwiched between the supporting substrate and the conductive sheet.After the posts are created, the bond pads may be electrically connectedto respective leads. Alternately, the conductive leads could be formedon the second surface of the support structure and coupled to each postthrough a conductive via. A highly conductive layer, such as gold, mayoptionally be plated on the surface of the posts to ensure a goodelectrical connection when the posts are coupled to contact pads on asupporting substrate.

The foregoing and other objects and advantages of the present inventionwill be better understood from the following Detailed Description of aPreferred Embodiment, taken together with the attached Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are perspective views illustrating the process ofsubtractively creating the interconnections according to one embodimentof the present invention.

FIGS. 2A and 2B are perspective views each showing the embodiment inFIG. 1B coupled to a compliant layer and a microelectronic deviceaccording to one embodiment of the present invention.

FIGS. 3A-C are perspective views each showing one possible shape of thesubtractively created interconnections according to the presentinvention.

FIG. 4A is an elevational view of a post and socket.

FIG. 4B is a perspective view of a post and socket.

FIG. 5 is a perspective view of a subtractively created interconnectionhaving an etch resistant, conductive cap thereon according to thepresent invention.

FIG. 6A is an elevational view of a post.

FIG. 6B is a top view of a brazing button and hole.

FIG. 7 shows a cross-sectional view of a microelectronic assembly, inaccordance with certain preferred embodiments of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring to FIGS. 1A and 1B, a top surface of a support structure 100is coupled to a conductive sheet 110. In the preferred embodiment of theinvention, the support structure is a flexible, but substantiallyinextensible, film preferably formed from a polymeric material, such asKapton™, of an approximate thickness between 25 microns and 75 micronsand is laminated to the second surface of the conductive sheet. However,the support structure could be comprised of many other suitablematerials and may further be semi-flexible or substantially rigid. Theconductive sheet 110 is preferably comprised of a conductive metal, suchas copper, copper alloys or phosphor bronze, among other materials.Portions of the conductive sheet are selectively removed by any suitablemeans to create a plurality of subtractively created, substantiallyrigid posts 130, as shown in FIG. 1B.

In the preferred embodiment, portions of the conductive sheet areremoved by first providing a photoresist mask on the surface of theconductive sheet and etching away the conductive sheet 110 around themask portions. This is preferably accomplished by coupling a photoresistlayer 120 to the top surface of the conductive sheet 110. Selectedportions of the photoresist layer 120 are then exposed and developedusing standard industry techniques, resulting in a plurality of etchresistant photoresistive portions 125 atop the conductive sheet 110. Aone sided etching process is then employed to remove portions of theconductive sheet 110 around the plurality of etch resistantphotoresistive portions 125 while substantially leaving the portionsbeneath the plurality of etch resistant photoresistive portions 125, asshown in FIG. 1B. The etch profile of features created from a conductivesheet, such as a metal foil, can be influenced by the process used toproduce them. The two most common methods of etching are batch immersionin an etchant solution and liquid etchant spraying or impingement. Inbatch etching, the features can be more uniformly created. Etchingproceeds isotropically removing metal at a basically uniform rate bothvertically and laterally. This results in creating posts havingsubstantially uniformly sloping vertical sides of approximately a 45°angle relative to the surface of the support structure. Etching normallyproceeds rather slowly in batch processing providing sufficient time toreplenish the active etchant solution to foil under the resist. Incontrast, a spray etching technique typically impinges the part at moreof a 90° angle, facilitating the etching of surfaces. exposed to theimpingement. While the etching process still progresses in a more orless isotropic fashion, the etch resistant photoresist portions 125 actas a shield causing the etching process to produce an etch profile whichforms “cooling tower” shaped posts 130 having a broad base which thinsas it reaches the vertical center of the post 130 and flares back outslightly as it reaches its apex. These features are caused by the“splash back” of the etchant solution against the walls of the emergingpost and can be more or less exaggerated by altering the pressure,concentration and or formula of the etchant within the bounds of thephotoresist's resistance to the etchant.

The height of each post will vary directly with the thickness of theconductive sheet 110, but typically will be in the range of 125 to 500microns. Because of their shape and rigidity, the conductive posts 130will resist deformation. A fine post connect pitch can therefore becreated without substantial fear that the posts 130 will be bent intoelectrical contact with each other. The possible pitch of the bumps isalso a function of the thickness of the sheet of conductive material.The thinner the conductive sheet, the finer the possible pitch of thebumps. Also, this process of creating the posts is cost and timeeffective when compared with methods which create each bump by platingor soldering. Further, the posts created with this subtractive processare extremely uniform and planar when compared to solder or plated bumpsbecause they are created from a single planar, conductive sheet. Thisensures that each of the bumps will make contact with respective contactpads on a supporting substrate, such as a printed wiring board, withoutthe exertion of undue pressure on the top surface of the microelectronicdevice.

The exterior surfaces of the posts may be optionally plated with ahighly conductive layer, such as gold, gold/nickel, gold/osmium orgold/palladium, or alternately plated with a wear resistant, conductivecoating such as osmium to ensure that a good connection is made when theposts are either soldered or socketed to a supporting substrate, asdescribed more fully below.

Referring now to FIG. 2A, a compliant layer 140 is coupled to the backsurface of the support structure 100. The compliant layer 140 istypically made of an elastomer material, such as the Dow Corning siliconelastomer 577 known as Silgard®. The compliant layer 140 is coupled tothe back surface of the support structure 100 by conventional stencilprinting techniques. The silicon elastomer used in the preferredembodiment is filled with about 5-10% of fumed silica in order to obtaina stiff consistency that allows the layer 140 to retain its shape afterthe stencil is removed. The silicon is then cured at a suitabletemperature. Typically, the thickness of the complaint layer is 150microns, plus or minus 12.5 microns. The compliant layer 140 mayalternately be replaced with a plurality of compliant pads 145 eachpositioned beneath a respective post, as shown in FIG. 2B. The pads 145are also typically stenciled on the back surface of the supportstructure 100 and the original stiff formulation of the elastomer allowseach individual pad 145 to retain its shape after the stencil has beenremoved. The exposed surface of the compliant layer is next engaged witha surface of a microelectronic device 150 having a plurality of bondpads 160 thereon.

Referring now to FIG. 2B, before the bond pads 160 can be connected tothe conductive posts 130, a method of electrically connecting the posts130 to the bond pads 160 must be supplied. One method includes providingetch-resistant conductive leads 170, such as copper leads which havebeen lithographically formed on the top surface of the support structure100 plated with gold prior to coupling the structure 100 to theconductive sheet 110. After the conductive sheet 110 has been reduced tothe conductive posts 130, shown in FIG. 2A, the etch resistantconductive leads may be connected to the bond pads 160 by any suitablemanner, such as wire bonding or by allowing the leads to extend beyondthe edge of the support structure such that they may be bent towards andthermosonically or ultrasonically bonded to a respective bond pad, asshown in FIG. 2A. An alternate method of creating a similar embodimentis to first plate a plurality of either one layer or a multi-layer etchresistant conductive leads, such as gold or gold/copper leads, to thebottom surface of the conductive sheet 110 prior to coupling theconductive sheet 110 and the support structure 100. Portions of theconductive sheet are then removed to create the conductive posts 130allowing the bond pads 160 to be electrically connected to the posts 130by the conductive leads. A further alternate solution involves formingthe leads on the second side of the support structure 100 and connectingthe posts through conductive vias extending from the first to the secondsurface of the support structure 100.

A further embodiment of the present invention, includes directlyattaching the support structure 100 to the microelectronic device suchthat each post is in electrical contact with a juxtaposed bond pad onthe microelectronic device. This is typically accomplished using aconductive via positioned beneath each of the posts and extending from afirst to a second surface of the support structure. The via may becreated by punching or laser ablating holes in the support structure andplating a conductive metal, such as copper into each of the holes. Ajoining layer, such as a gold/tin or silver/tin alloy, is next typicallycoupled to the copper. The joining layer will weld to its respectivebond pad under the correct temperature, pressure or vibration stresses.

As stated above, the shape of the posts 130 can depend on the processused to remove the surrounding conductive material. However, the shapeof the etch resistant photoresist portions 125 in FIG. 1A may alsoproduce different shaped posts from the conductive sheet material. Forexample, FIG. 3A shows a substantially in the form of a surface ofrevolution which is the result of using circular resist portions 180 onthe conductive sheet 110. Square resist portions 190 will produce a posthaving four slightly concave, rounded sides meeting at slightly roundededges, as shown in FIG. 3B. Triangular resist portions 200 will producea post having three slightly concave, rounded sides meeting at slightlyrounded edges, as shown in FIG. 3C. Each of these photoresist portionsproduce the “cooling tower” shape shown if a spray etching process isused. If a batch immersion process is used, the resulting posts willhave more linearly sloping vertical walls and slightly sharper comers.

The peaks of the posts 130 may then be coupled to the contact pads onthe supporting substrate by any suitable means, such as directlysoldering the posts to the contact pads or inserting them into socketsattached to the substrate. The “cooling tower” shape created by sprayetching makes for a more reliable leaf-spring socket connection becauseits peak has a larger diameter than its middle section, as shown in FIG.4A. The peak of the post will thus provide resistance to being pulledout of the socket in response to forces acting in the lengthwise planeof the posts. The vertical comers on the posts shown in FIGS. 3B and 3Cpartially inserted into round socket holes or vias also makes for a morereliable, force fit, separable, electrical connection with each sockethole contact, as shown in FIG. 4B.

FIG. 5 shows a further embodiment in which the photoresist layer 120, inFIG. 1, is replaced with a plurality of metallic portions 210 of ageometry similar to the photoresist portions (180/190/200) in FIGS.3A-C. Typically, the metallic portions 210 are comprised of an etchresistant metal, such as nickel. The conductive layer may then be etchedaround the metallic portions 210 leaving the post capped with aconductive top. This conductive top may then be plated with a highlyconductive layer, such as gold or a gold alloy. This conductive topfurther increases the reliability of an electrical connection when theposts are inserted into the type of socket shown in FIG. 4A. In analternate embodiment, solder can also be used as an etch resist. Afterthe posts are created, the solder can then be reflowed to create asolder coated post. If the solder is reflowed after the post has beeninserted into a test socket, it will create a more permanent electricalconnection with the socket.

FIGS. 6A-B show a still further embodiment having a brazing button 220extending through brazing hole in a removable support structure 230. Thebrazing button is used to attach the post directly to a bond pad on amicroelectronic device and is typically comprised of a metallic alloywhich will attach easily and provide a good electrical connection withits respective bond pad, such alloys include gold-tin, bismuth-tin,gold-silicon, or tin-silver. FIG. 6B shows one embodiment of a brazinghole 240 which allows for expansion of the brazing button when it isheated to attach to the chip bond pad. The removable support structure230 is comprised of a material which may be removed by any suitablemeans after the posts have been attached to the bond pads, such as usinga paper or water soluble polymeric support structure which may besprayed with water and peeled off.

FIG. 7 shows a microelectronic assembly, in accordance with the anotherpreferred embodiment of the present invention. The microelectronicassembly includes a substrate 310, such as a dielectric substrate or aflexible dielectric substrate, having conductive posts 330 projectingfrom a first surface thereof. The tips of the conductive posts 330 areelectrically interconnected with bond pads 360 provided on amicroelectronic element 350, such as a semiconductor chip. Themicroelectronic assembly includes a compliant layer 340 overlying asecond surface of the substrate 310.

One skilled in the art will appreciate that the subtractively createdposts described herein could be used for many other purposes besidesconnecting microelectronic devices to supporting substrates withoutdeparting from the spirit of the present invention. Further, if the topsurfaces of the posts are sufficiently wide, a cupped portion could beprovided thereon to receive bumps or solder balls on the surface of asupporting substrate. Having fully described several embodiments of thepresent invention, it will be apparent to those of ordinary skill in theart that numerous alternatives and equivalents exist which do not departfrom the invention set forth above. It is therefore to be understoodthat the present invention is not to be limited by the foregoingdescription, but only by the appended claims.

1. A microelectronic unit comprising: (a) a support structure includinga dielectric having oppositely-directed first and second surfaces and aplurality of substantially rigid posts protruding parallel to oneanother in a direction beyond said first surface of said supportstructure, each post having a top surface remote from said supportstructure, said top surfaces being substantially coplanar with oneanother; (b) a microelectronic device having a surface with bond pads,said microelectronic device overlying said second surface of saidsupport structure with said surface facing toward said supportstructure; and (c) connections electrically connecting said posts withsaid bond pads.
 2. The unit as claimed in claim 1, wherein the supportstructure is a flexible dielectric structure.
 3. The unit as claimed inclaim 1 wherein said connections include wire bonds.
 4. The unit asclaimed in claim 1 wherein said connections include electricallyconductive leads on said support structure electrically connected tosaid posts.
 5. The unit as claimed in claim 1 wherein a height of saidtop surfaces beyond said first surface is less than or equal to 500microns.
 6. The unit as claimed in claim 5, wherein said height is atleast 125 microns.
 7. An assembly comprising a unit as claimed in claim1 and a substrate having electrically conductive features, said postsbeing connected to said electrically conductive features.
 8. The unit asclaimed in claim 1, wherein said posts protrude from the first surfaceof said support structure.
 9. The unit as clamed in claim 1, whereinsaid support structure includes a plurality of dielectric layers.